[X] My Assistant

[ljk4-1]

Astrophysics, abstract
astro-ph/0601709

From: Anthony Remijan [view email]

Date: Tue, 31 Jan 2006 16:02:51 GMT (39kb)

A Bima Array Survey of Molecules in Comets Linear (C/2002 T7) and Neat (C/2001 Q4)

Authors: Anthony J. Remijan, D. N. Friedel, Imke de Pater, M. R. Hogerheijde, L. E. Snyder, M. F. A'Hearn, Geoffrey A. Blake, H. R. Dickel, J. R. Forster, C. Kraybill, L. W. Looney, Patrick Palmer, M. C. H. Wright

Comments: Accepted for Publication in the Astrophysical Journal

We present an interferometric search for large molecules, including methanol, methyl cyanide, ethyl cyanide, ethanol, and methyl formate in comets LINEAR (C/2002 T7) and NEAT (C/2001 Q4) with the Berkeley-Illinois-Maryland Association (BIMA) array. In addition, we also searched for transitions of the simpler molecules CS, SiO, HNC, HN13C and 13CO . We detected transitions of methanol and CS around Comet LINEAR and one transition of methanol around Comet NEAT within a synthesized beam of ~20''. We calculated the total column density and production rate of each molecular species using the variable temperature and outflow velocity (VTOV) model described by Friedel et al.(2005).Considering the molecular production rate ratios with respect to water, Comet T7 LINEAR is more similar to Comet Hale-Bopp while Comet Q4 NEAT is more similar to Comet Hyakutake. It is unclear, however, due to such a small sample size, whether there is a clear distinction between a Hale-Bopp and Hyakutake class of comet or whether comets have a continuous range of molecular production rate ratios.

http://arxiv.org/abs/astro-ph/0601709

[Guest_AlexBlackwell_*]

Published online today in Science Express:

Exposed Water Ice Deposits on the Surface of Comet Tempel 1
J. M. Sunshine, et al.
Published online February 2, 2006; 10.1126/science.1123632 (Science Express Reports)
Abstract

[Guest_Sunspot_*]

From New Scientist too:

http://www.newscientistspace.com/article.ns?id=dn8670

[The Messenger]

From New Scientist too:

http://www.newscientistspace.com/article.ns?id=dn8670

The consensus model of a comet leading up to the Deep Impact experiment is no longer valid, says Don Yeomans at NASA’s Jet Propulsion Laboratory in Pasadena, California, US, a member of the mission science team. "It's certainly not a dirty iceball or an icy dirtball," he told New Scientist. "It's a very, very weak, dusty structure with interior ices."

Nice to see a flawed model finally laid to rest.

Edited to add: I am not sure where the conclusion that it is a weak structure comes from: We saw dust, we did not see the impact crater, the surface LOOKs like in has survived cratering impacts implies a resilent, hard layer under a lot of fine dust.

The mass of the nucleus was determined, according to an earlier article, by monitoring the settling of dust particles and calculating GM. Static electricity can exist in space, and therefore gravity may not be the only force involved in calculating the mass of the nucleus - even so the error bars reported are ~ 50%. If the mass is so low, how can it be so dusty?

I stand by my interpretation that the dust cloud MAY be the result of a timpanic response by a relatively resilient surface, revealing nothing about the nucleus interior.

[ljk4-1]

SPOTTED! ICE ON COMET NUCLEUS (Space & Astronomy News, 3/2/06)

Ice has been detected on the nucleus, or solid body, of a comet for the
first time, researchers report.

http://www.abc.net.au/science/news/space/S...ish_1561541.htm

[Guest_RGClark_*]

A report to be presented to the 37th Lunar and Planetary Science conference will argue that the Deep Impact results can not actually distinguish between the gravity-dominated or strength-dominated scenarios for the comet's make-up, despite the earlier reports the comet's make-up was gravity dominated and therefore very porous:

GRAVITY OR STRENGTH? AN INTERPRETATION OF THE DEEP IMPACT EXPERIMENT.
K. A. Holsapple1, K. R. Housen2
1Dept. of Aeronautics and Astronautics, University of Washington 352400, Seattle, WA 98195
2 Physical Sciences, MS 2T-50, The Boeing Co., P.O. Box 3707, Seattle WA 98124
Lunar and Planetary Science XXXVII (2006) 1068.pdf
http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1068.pdf


If the comet was in fact strength-dominated then another explanation has to be found for its low density, rather than porosity.
One possibility is that the comet has hollow voids. This would be consistent with the theory that liquid water existed on comets early in the Solar Systems history due to radiogenic heating and this liquid water boiled off or evaporated over time:


Deep Impact Spectral Analysis Results, carbonates and amino acid precursors.
http://www.unmannedspaceflight.com/index.p...975&#entry20975


Bob Clark

[Guest_RGClark_*]

Lisse et.al. report on their detection of carbonate and clay from Deep Impact:

Spitzer Space Telescope Observations of the Nucleus and Dust of Deep Impact Target
Comet 9P/Tempel 1
C.M. Lisse1 and the Deep Impact Spitzer Science Team. 1 JHU-APL,
11100 Johns Hopkins Road, Laurel, MD 20723 ****@jhuapl.edu.
http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1960.pdf

They have submitted this for publication in Science.


- Bob

[The Messenger]

Lisse et.al. report on their detection of carbonate and clay from Deep Impact:

Spitzer Space Telescope Observations of the Nucleus and Dust of Deep Impact Target
Comet 9P/Tempel 1
C.M. Lisse1 and the Deep Impact Spitzer Science Team. 1 JHU-APL,
11100 Johns Hopkins Road, Laurel, MD 20723 ****@jhuapl.edu.
http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1960.pdf

They have submitted this for publication in Science.
- Bob

I have a problem with the logic here:

The overall temperature of the outflowing material appears to be very mild for such a violent event, about 340 degrees Kelvin, similar to the temperatures measured for the surface of the nucleus and small, semi-absorbing dust grains in the Sun's radiation field at 1.5 AU. There is simply not enough kinetic energy delivered by the impactor - approximately 2x107 kJoules - to heat the approximately 107 kg of material released by the impact more than a few degrees Kelvin. Thus the temperature excursions due to the impact are too low to alter the bulk of the ejected dusty materials...

We have measured the thermal capacity of Tempel 1, and it is very low: The 'dark' side cools almost immediately as the nucleus rotates. So the surface is not unlike a volcanic field desert in Idaho, where in mid August, the temperature a mere 4m under the surface can be less than 0C.

Now check out Lisse's conclusion:

...Thus Spitzer was directly measuring pristine cometary material from inside the comet, material that has been locked away since the beginnings of the solar system.

Why would the temperature of the material inside the comet be with a few degrees of the the surface at 1.5 AU? The Comet spends most of its existance at much greater distances, so I would expect, from the known properties of the surface, that the inner temperture should be much cooler, not within a few degrees of the surface temperature at the time of impact. A more reasonable conclusion would be that the dust lifted by the impact was very near the surface.

[edstrick]

The dust is so fine and powdery that once ejected, it very rapidly reaches thermal equilibrium with sunlight. The technical term is that it has extremely low thermal inertia.

In addition, partly determining the low thermal inertia, the dust has extremely low thermal conductivity. This means that the diurnal thermal heating/cooling cycle is very shallow, and even the annual (orbital) thermal heating/cooling cycle doesn't penetrate far. Centuries-long heating in a low-semi-major-axis orbit will penetrate only 5 or 10 times the depth of the annual thermal cycle. Deeper areas have hardly changed temperature in a thousand years.

These depths are shallow relative to the depth of excevation of any plausible crater whatever, so much of the material is inferred to have been relatively pristine.

[The Messenger]

The dust is so fine and powdery that once ejected, it very rapidly reaches thermal equilibrium with sunlight. The technical term is that it has extremely low thermal inertia.

In addition, partly determining the low thermal inertia, the dust has extremely low thermal conductivity. This means that the diurnal thermal heating/cooling cycle is very shallow, and even the annual (orbital) thermal heating/cooling cycle doesn't penetrate far. Centuries-long heating in a low-semi-major-axis orbit will penetrate only 5 or 10 times the depth of the annual thermal cycle. Deeper areas have hardly changed temperature in a thousand years.

These depths are shallow relative to the depth of excevation of any plausible crater whatever, so much of the material is inferred to have been relatively pristine.

Agreed the dust normally reaches thermal equalibrium quickly, but Lisse implies they would have been able to detect it if the ejecta was hot, and it was not: Remember, we are working in a vacuum here, and the only mechanism for transferring heat is radiation. It would take ejecta from inside the comet a significant amount of time to reach thermal equalibrium using radiation transfer mechanisms only, especially with the forground dust shading background dust relative to the only significant source of radiation: the sun at 1.5 AU. I haven't run the numbers, but my gut tells be using radiation transfer to equilibrate the temperature of a thick plume of dust could take days.

I don't see how, given the data in-hand, an unqualified statement can be made that the dust was 1) from the interior 2) pristine.

[ljk4-1]

Silicates, like glass, require heat to transform into crystals. The gem-like
particles can be found in the Milky Way in limited quantities around certain
types of stars, such as our sun. On Earth, they sparkle in sandy beaches,
and at night, they can be seen smashing into our atmosphere with other
dust particles as shooting stars.

Recently, the crystals were also observed by Spitzer inside comet Tempel 1,
which was hit by NASA's Deep Impact probe

http://www.spitzer.caltech.edu/Media/relea...8/release.shtml

[antoniseb]

my gut tells be using radiation transfer to equilibrate the temperature of a thick plume of dust could take days.

My gut tells me that the speed at which the dust can radiate IR away depends on the optical depth of the plume. The only reason that a single grain would not cool down as fast as it would far away from other objects is by receiving IR from its neighbors. How many steps does the photon have to make to get out of the cloud on average. I'm guessing that after the first few minutes the answer is between zero and one step (much closer to zero). This means that the dust should cool off pretty quickly, perhaps getting down to a few degrees above the final temperature within minutes.

[Bob Shaw]

Yes; dust is practically all surface area, so radiation, chemical or whatever processes are *fast*!

Bob Shaw

[The Messenger]

My gut tells me that the speed at which the dust can radiate IR away depends on the optical depth of the plume. The only reason that a single grain would not cool down as fast as it would far away from other objects is by receiving IR from its neighbors. How many steps does the photon have to make to get out of the cloud on average. I'm guessing that after the first few minutes the answer is between zero and one step (much closer to zero). This means that the dust should cool off pretty quickly, perhaps getting down to a few degrees above the final temperature within minutes.

But my argument is that the dust plume should be much colder, not hotter, if the dust originated 5-50 meters into the core of the nucleus. This dust would take considerable time to reach equalibrium, as the dust at the solar facing side of the plume would shade the dark side of the plume. These IR transfers, at these very low temperatures would take considerable time to reach equalibrium, even for very small particles.

Lisse said the energy from the probe would have only heated the bulk of dust emitted a few degrees K. If Tempel I is truly 90% void (by volume) blackened sand with low thermal capacity, the Earth-like material it most resembles is volcanic pumice and soil fields. The temperature gradiant in these fields is as high as 20degK/m. Since comets spend most of their life in the Kuiper belt, I would expect the core of the nucleus to be 70deg K colder than the surface @ 1.6AU.

If the dust in the plume was excavated from more than a few meters below the surface, I would expect an obvious negative temperature gradiant across the plume that would take at least hours, if not days to equilabrate. Unless there is evidence of this, it is more reasonable to assume most of the dust plume originated near or at the surface, which would also help explain why the moisture content is so low.

It would be helpful to know how the particle size of the dust was calculated - this is not a trivial or exacting calculation for an unknown particulate in broadband light.

[ljk4-1]

The Deep Impact team wants to go back to Comet Tempel 1 with the
surviving flyby bus and a new spacecraft. They want not only that
look into the crater they made last July 4 but to explore the comet
overall in detail as well as another one never seen up close before.

To quote:

"The proposed new missions are called DeepR and DIXI. DeepR (Deep-Rosetta) would clone the Deep Impact mission, building identical flyby and impactor spacecraft and targeting comet 67P/Churyumov-Gerasimenko (C-G), the destination of the European Space Agency's currently-in-route Rosetta mission.

"DIXI, which stands for Deep Impact eXtended Investigation, would use the surviving Deep Impact spacecraft and its three working instruments (two color cameras and an IR spectrometer) for a flyby of Comet Boethin in December 2008."

The article:

Deep Impact Team Want To Go Back With A Bang

http://www.spacedaily.com/reports/Deep_Imp...ith_A_Bang.html